The demand for smaller packaging of electronic components continues to drive the development of smaller chip scale packages (CSP's), bumped die, and other similar devices having solder bumps, ball grid arrays (BGA's), or the like. As a result, spacing (or "pitch") between adjacent solder balls on bumped devices has steadily decreased. Typical requirements for ball pitch have decreased from 1.27 mm to 0.5 mm or less, and the trend continues.
FIG. 1 is a side elevational view of a typical bumped device 10 (CSP, bumped die, etc.) mounted on, for example, a printed circuit board 20. The bumped device 10 includes a plurality of solder balls 12 attached to a plurality of ball pads (not shown) which are formed on a die 14. Each solder ball 12 has an outer edge 16 that aligns with a corresponding contact pad 18 on the printed circuit board 20. A conductive lead 22 is attached to each contact pad 18. Ideally, the outer edge 16 of each solder ball 12 contacts the corresponding contact pad 18 during assembly of the bumped device 10 with the printed circuit board 20, completing the electrical circuit between the conductive leads 22 and the die 14.
The height and width of the solder bumps 12 on the bumped device 10 are not precisely uniform. Variation of the solder bump height and width depends on several factors, including variation in size of the original unattached solder balls, variation in the sizes of the ball pads, and differences in the attachment process.
As the demand for smaller packaging continues, however, CSP reliability concerns arise. For example, using typical manufacturing methods and solders, the nominal variation between the tallest and shortest balls (shown as the distance d on FIG. 1) is presently about 60 microns (.mu.m). Therefore, when the device 10 is placed on a flat surface resting on the solder balls, the three tallest balls or bumps define the seating plane of the device, and the smaller balls do not touch the corresponding contact pads of the printed circuit board or test interposer.
During assembly, and in some cases during testing, a moderate compression force may be applied to the bumped device 10 to drive the outer surfaces 16 of the solder balls 12 into contact with the contact pads 18 of the printed circuit board or test interposer 20. Typically, the compression force needed to bring the solder bumps into contact with the contact pads varies between 30 grams and 2000 grams depending upon the manufacturing or test process involved. The applied compression force should be kept to a minimum, however, because larger forces may damage the circuitry of the die 14, the CSP solder balls, or the test interposer.
One approach to the problem is to mount the contact pads 18 of the test interposer 20 on micro-springs. As the tallest solder bumps engage the micro-spring mounted contact pads, the micro-springs are compressed, allowing the shorter solder balls to engage the corresponding contact pads. Numerous micro-spring contact pad models are available as shown and described in Robert Crowley's article in Chip Scale Review published May 1998, p. 37, incorporated herein by reference. Although desirable results may be achieved with such devices, micro-spring mounted contact pads 18 are very expensive, relatively difficult to maintain, and may excessively damage the solder ball itself.
During assembly of the bumped device 10 with the printed circuit board 20, some of the shorter solder balls may not solder to their associated contact pads during the reflow process. In the past, to increase the numbers of solder balls making contact with the contact pads during reflow, the volume of the solder balls was increased. As packaging sizes and pitch requirements continue to decrease, however, the volume of the solder balls must be reduced accordingly, and thus, the percentage of balls that will not attach to the contact pads during reflow increases. Again, if considerable force is applied during assembly, the CSP or the printed circuit board 20 may be damaged.